Huge amounts of sewage sludge produced globally is a substantial environmental threat and require rational handling. Application in agriculture is an economical and relatively simple method of sludge management, however, it is associated with restrictions regarding metals content. According to EU regulations, the total amounts of Cd, Cu, Cr, Pb, Ni, and Zn have to be analyzed by the AAS technique requiring effective destruction of the organic matrix. Currently used methods of sewage sludge digestion may be biased when applied without optimization. The aim of the presented work was to evaluate the efficiency of the organic substances destruction in either raw or stabilized sludge. Three mineralization procedures were evaluated, namely: (A)—drying and microwave digestion; (B)—ignition and microwave digestion; (C)—drying and conventional digestion. For matrix destruction, a mixture of concentrated HNO3 and HCl (3:1 v/v) was used. Metals were determined by flame atomic absorption spectrometry (FAAS). No limits of metal concentration were overdrawn. Generally, the method (B) was the most effective. Results obtained for Cu, Cd, and Zn after digestion by method (A) and (B) were comparable. Methods (B) and (C) yield complete decomposition of the matrix. As result, the precision of measurement substantially increases.